1. Field of the Invention
The present invention relates to a color filter substrate for a liquid crystal display device and a liquid crystal display device including the color filter substrate. In particular, the present invention relates to a color filter substrate optimized for driving liquid crystals by an oblique electric field generated by applying a voltage between a transparent conducting film disposed on a color filter substrate and first and second electrodes provided on an array substrate side and a liquid crystal display device including the color filter substrate.
2. Description of the Related Art
In recent years, still higher quality, lower prices, and lower power consumption of slim display devices such as liquid crystal displays are demanded. Regarding color filters for liquid crystal display devices, requests fitting to higher quality display such as adequate color purity, high contrast, and flatness are submitted.
The liquid crystal alignment method such as VA (Vertically Alignment), HAN (Hybrid-aligned Nematic), TN (Twisted Nematic), OCB (Optically Compensated Bend), and CPA (Continuous Pinwheel Alignment) and the liquid crystal drive method have been proposed and displays with a wide angle of visibility/fast response have thereby been commercialized.
In a liquid crystal display device of the VA system that can more easily be adapted to a wide angle of visibility and fast response by the liquid crystals aligned in parallel with the substrate surface formed of glass or the like or the HAN system that is effective for a wide angle of visibility, still higher levels of requests are made regarding flatness of color filters (uniformity of thickness and reduction of irregularities on the color filter surface) and electric properties such as the dielectric constant. For such a high-quality liquid crystal display, the main subject is a technology to slim the thickness of liquid crystal cells (thickness of the liquid crystal layer) to reduce coloring in oblique direction visual recognition. In the VA system, various improved modes such as MVA (Multi-Domain Vertically Alignment), PVA (Patterned Vertically Alignment), VAECB (Vertically Alignment Electrically Controlled Birefringence), VAHAN (Vertical Alignment Hybrid-aligned Nematic), and VATN (Vertically Alignment Twisted Nematic) are under development. In a liquid crystal display device of a longitudinal electric field system like the VA system in which a drive voltage is applied in the thickness direction of liquid crystal, the main subject includes a faster liquid crystal response, technology of a wider angle of visibility, and higher transmittance. The MVA technology is a technology to secure a wide angle of visibility by providing a plurality of liquid crystal alignment regulating structures called ribs or slits to form liquid crystal domains between these ribs and also to form domains in a plurality of alignment directions to solve the problem of unstable vertically aligned liquid crystals when a liquid crystal driving voltage is applied (the direction of inclination in which liquid crystals initially aligned vertically with respect to the substrate surface is less likely to be determined when a voltage is applied). Japanese Patent No. 2947350 discloses a technology to form a liquid crystal domain by using first and second or alignment regulating structures (ribs).
If the liquid crystal has negative dielectric constant anisotropy, more specifically, a liquid crystal positioned between two ribs made of resin formed on a color filter or the like attempts, for example, to incline in a direction perpendicular to the ribs to be aligned in parallel with the substrate surface in a plane view when a drive voltage is applied. However, the inclination direction in which the liquid crystals in the center between two ribs is not uniquely determined regardless of the application of voltage and may take a spray alignment or bend alignment. Such an alignment disorder of liquid crystal leads to a rough liquid crystal display or display unevenness. Also in the MVA system, it is difficult to finely control the amount of inclining liquid crystals by a drive voltage including the above problem and also the halftone display has weaknesses.
To solve such problems, Japanese Patent No. 2859093 and Japanese Patent No. 4459338 disclose a technology that uses a transparent conducting film (a transparent electrode, display electrode, or third electrode) on the side of a filter substrate and first and second electrodes on the side of an array substrate to control a vertically aligned liquid crystal by an oblique electric field generated by a voltage being applied to these electrodes. Japanese Patent No. 2859093 uses a liquid crystal of negative dielectric constant anisotropy and Japanese Patent No. 4459338 describes a liquid crystal of positive dielectric constant anisotropy.
As shown in Japanese Patent No. 2859093 and Japanese Patent No. 4459338, the method of using the first, second, and third electrodes to control the liquid crystal alignment by an oblique electric field is very effective. The inclination direction of the liquid crystals can be set by the oblique electric field. Moreover, the oblique electric field makes the control of the amount of the liquid crystal inclination easier, which has an important effect on the halftone display.
However, even such a technology has insufficient measures against disclination of liquid crystal. The disclination is a problem arising in a pixel (the pixel is the minimum unit of liquid crystal display and is synonymous with the pixel described in the present invention) in which regions of different optical transmittances arise due to an unintended alignment disorder of liquid crystal or non-alignment.
According to Japanese Patent No. 2859093, an alignment control window without transparent conducting film is provided in the pixel center of the counter electrode (third electrode) to fix the disclination in the pixel center. However, no improvement proposal of the disclination in the pixel periphery is disclosed. Though the disclination in the pixel center can be fixed, no proposal for minimizing the disclination is shown. Further, no technology to improve responsiveness of liquid crystal is described.
Japanese Patent No. 4459338 is preferable because a dielectric layer is stacked on the transparent conducting film (transparent electrode) and the effect of the oblique electric field is enhanced for the stacked dielectric layer. However, as shown in FIG. 7 of Japanese Patent No. 2859093, a vertically aligned liquid crystal remains in the pixel center and pixel periphery, causing a problem of lower transmittance or aperture ratio. If a liquid crystal of positive dielectric constant anisotropy is used (Japanese Patent No. 2859093 does not disclose any description/embodiment of a liquid crystal of negative dielectric constant anisotropy), it is difficult to improve transmittance due to the disclination in the pixel center. Thus, the technology is hard to adopt for a transflective liquid crystal display device.
The basic configuration of a liquid crystal display device of the VA system or TN system is normally a configuration in which a liquid crystal is sandwiched between a color filter substrate including a common electrode and an array substrate including a plurality of pixel electrodes (for example, transparent electrodes electrically connected to a TFT element and formed in a comb-like pattern shape) driving the liquid crystal. In this configuration, the liquid crystal is driven by applying a drive voltage between the common electrode on the color filter and the pixel electrodes formed on the side of the array substrate. A thin film of conductive metal-oxides such as ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), and IGZO (Indium Gallium Zinc Oxide) is used as the pixel electrode and the transparent conducting film as the common electrode on the color filter surface.
FIG. 2 of Jpn. Pat. Appln. Publication No. 5-26161 discloses a technology to disclose the configuration of a color filter in which a blue pixel, green pixel, and red pixel are formed on a transparent conducting film. Also, a technology, though using a plurality of stripe electrodes and a liquid crystal of positive dielectric constant anisotropy, that forms a color filter on a transparent electrode (transparent conducting film) is described in Japanese Patent No. 2859093 (for example, FIGS. 7 and 9 of the patent literature).
A technology to improve luminance or lightness or extend the chromaticity range to obtain a higher quality and more dynamic display is disclosed by Jpn. Pat. Appln. KOKAI Publication No. 2010-9064, Japanese Patent No. 4460849, and Jpn. Pat. Appln. KOKAI Publication No. 2005-352451. Also, a technology of 4-color display by adding a yellow pixel or white pixel, in addition to the blue pixel, green pixel, and red pixel is known.
However, it is necessary for these technologies to provide another pixel such as a yellow or white pixel and an active element (TFT) to drive the pixel and still one color layer to form a color filter are needed, leading to higher costs due to increased processes. In addition, it is necessary to inhibit or turn off the display of white or yellow pixels in a gradation display range in which the display of yellow or white that is bright in intensity is not needed, causing a problem of being unlikely to lead to an effective increase in luminance. Further, in the reflective type display, a problem that the display takes on a yellow tinge is posed (to suppress the yellow tinge, for example, a special blue filter disclosed by Jpn. Pat. Appln. KOKAI Publication No. 2005-352451 is needed).